DVB is defined in particular in the standards ETSI EN 301 192 (entitled “Digital Video Broadcasting (DVB); DVB specification for data broadcasting” and TR 101 190 (entitled “Digital Video Broadcasting (DVB); Implementation guidelines for DVB terrestrial services; Transmission aspects”). DVB-H is specified in particular in the standards ETSI EN 302 304 (entitled “Digital Video Broadcasting (DVB); Transmission System for Handheld Terminals (DVB-H)” and TR 102 377 (entitled “Digital Video Broadcasting (DVB); DVB-H Implementation Guidelines”).
According to the prior art, a DVB-H television service (for example, live television or file server or VOD (“Video On Demand”) server) can be downloaded in streaming mode or in file mode. A protocol known as FLUTE (“File Delivery over Unidirectional Transport”) is described in the document entitled “RFC 3926, FLUTE—File Delivery over Unidirectional Transport” (by T. Paila, M. Luby, R. Lehtonen, V. Roca, R. Walsh). This protocol is used to deliver files and other discrete objects within a session. It is used in particular for the transmission of files on DVB-H. In a FLUTE session, a plurality of transport objects can be transferred; these transport objects represent, for example, files or “File Delivery Tables” (FDT). Each object is identified by its URI and its version is identified by its TOI (“Transport Object Identifier”) in the form of an unsigned integer. The value 0 is reserved for an FDT, while the other values can be used for other objects. Before files are despatched, a description of these files is transmitted in an FDT according to the FLUTE protocol. For each file, the FDT associates its name (URI) with the TOI value. A description table is transmitted in the form of FDT instances, which are XML structures. Each FDT instance transports all or part of the current FDT, for example information on a subset of a file to be transmitted. Each FDT instance transports an FDT instance identifier, which is an unsigned 20-bit number. These identifiers are incremental and can therefore be used to distinguish recent instances from older instances from the same session. An FDT instance is associated with one or more files and contains an “expires” attribute, which defines an expiry time of an FDT instance. Here, the term “file” corresponds to any object transported according to the FLUTE protocol, with the exception of FDT instances. According to the FLUTE specifications, the receiver must not use a received FDT instance to interpret packets received after the FDT instance expiry time.
FIG. 1 shows schematically a file distribution service broadcast in a DVB-CBMS (“Convergence of Broadcast and Mobile Services”) context (or DVB specified in the transfer of IP (“Internet Protocol”) data on DVB-H: “IP Datacast over DVB-H protocol: Content Delivery Protocols (CDP)”). A file server 100 broadcasts a first FDT 110 which is transmitted via the FLUTE protocol over IP on a DVB-H network to a terminal 101. The first FDT 110 comprises a URI (“Uniform Resource Identifier”) which enables identification of the file (and more generally the object) which is to be transmitted. The terminal receives the first FDT and records the description of the file which is to be broadcast according to this first FDT. The file server 100 then broadcasts data in the form of one or more symbols corresponding to parts of files in the form of successive frames 111 to 113 to the terminal 101 via the DVB-H network. By way of illustration, it is assumed that the file server 100 broadcasts a second FDT 120 with the same URI to the terminal 101 in the same time interval as the terminal 101 transmits a message 114 (via an Internet channel uplink, for example) to the file server 100. According to a different scenario, the message 114 is transmitted if the second FDT 120 is not correctly received or is not received by the terminal (which may result in system malfunctions). The message 114 contains the URI corresponding to the two FDTs and an identifier of the fragment to be retransmitted. It enables the terminal 101 to request one or more symbols which have not been correctly received or have not been received (or fragments) from the file server 100 via DVB-H. The second FDT 120 does not have to be received by the terminal 101. On receiving the message 114 relating to the first FDT, the server 100 forwards a frame 121 comprising symbols which have the same identifier as the requested symbols, but which correspond to the second FDT and not to the first FDT. The disadvantage of this technique is therefore that it generates the transmission of errored symbols, which may result in terminal malfunctions.